The embodiments described herein relate generally to emissions treatment systems and, more particularly, to a heat recovery steam generator for use in reducing NO2 formation in the exhaust path of a combustion system and for regenerating a catalytic material that facilitates the reduction in NO2 formation.
During the combustion of natural gas and liquid fuels, pollutants such as, but not limited to, carbon monoxide (CO), unburned hydrocarbons (UHC), and oxides of nitrogen (NOx) emissions, may be formed and/or emitted into an ambient atmosphere. In general, CO and UHC may be formed during combustion conditions at lower temperatures and/or during combustion conditions when an insufficient amount of time to complete a reaction is available. In contrast, NOx is generally formed during combustion conditions at higher temperatures. At least some known pollutant emission sources include industrial boilers and furnaces, reciprocating engines, gas turbine engines, and/or steam generators.
Modern air quality regulations increasingly mandate reduced emission levels for power generation plants, while also requiring increased fuel efficiency requirements. To comply with stringent emission control standards, it is desirable to control NOx emissions by suppressing the formation of NOx emissions. Oxides of nitrogen include nitric oxide (NO) and nitrogen dioxide (NO2), which is known to produce a visible yellow plume from exhaust stacks and that is alleged to contribute to the creation of “acid rain.” However, known combustion controls may provide only limited emissions control and may prove inadequate in satisfying the increased standards and the often-conflicting goals, such that further improvements of post-combustion exhaust gas treatment systems are desirable.
One known technology for use in controlling NOx in stack emissions is selective catalytic reduction (SCR). In an SCR system, flue gases from power generation plants often have a net oxidizing effect due to a high proportion of oxygen that is provided to ensure adequate combustion of a hydrocarbon fuel. Thus, NOx that is present in the flue gas may be reduced to nitrogen and water with great difficulty. An SCR element may be used to mix anhydrous ammonia with the flue gas, and the gases are channeled over a suitable reduction catalyst at a suitable temperature prior to being released into the atmosphere. However, the reaction rate over the catalyst is dependent on the inlet gas temperature as such the rate of NOx destruction is insufficient until the flue gas is heated to the suitable temperature. Accordingly, during transient phases, such as during startup operations, the SCR element generally does not reduce the NOx to a desired level as the flue gas temperature is to low.
Another approach to NOx removal is adsorption by a downstream medium and subsequent removal of the NOx from the downstream medium. For example, NO2 is relatively water-soluble, which enables its removal by adsorption using a downstream aqueous solution in a flue gas desulfurization (FGD) unit. However, NO is less soluble than NO2 and other NxOy compounds in such solutions. Accordingly, power generation systems can use an oxidation catalyst to oxidize NO to a more soluble NxOy component, such as NO2 and N2O5. Moreover, NO oxidation is generally at its most effective at low temperatures. However, sulfur dioxides (SOx) and other contaminants can foul the oxidation catalyst over time, which reduces the NO oxidation yield.